RFR: 8304912: Use OperatingSystem enum in java.desktop module [v3]

[Roger Riggs]

> Update classes in the java.desktop module to use the jdk.internal.util.OperatingSystem enum instead of the `os.name` system property to select OS specific behaviors.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright
  Replaced a few more uses of os.name.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13213/files
  - new: https://git.openjdk.org/jdk/pull/13213/files/7f93ecbf..8f0b918e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13213&range=01-02

  Stats: 9 lines in 3 files changed: 2 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/13213.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13213/head:pull/13213

PR: https://git.openjdk.org/jdk/pull/13213